Update the cadence form UI

What does this MR do and why?

This MR contains a minor UI refactor for the Iterations Cadence form.

A further refactor/change to the UI is required to work towards the final design #363666[Cadence_form.png] after more backend/frontend works are completed.

Related to #367493 (closed)

Screenshots or screen recordings

Before	After (1)	After (2)

How to set up and validate locally

1. Ensure to enable the iteration cadences FF (it's enabled by default.)

   Feature.enable(:iteration_cadences)

2. Visit any group (not project.) Via the left menu bar, navigate to the Iterations and click New iterations cadence.

Screen_Recording_2022-07-12_at_18.31.37

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.